1. Field of the Invention
The invention relates to a process for surface activation and/or devulcanization of sulfur-vulcanized rubber particles.
2. Background and Related Art
Different processes are known from DE 4425049 C1, DE 19607281 A1, U.S. Pat. No. 5,506,283 for activation of comminuted scrap rubber and waste rubber. These processes are based either on physical or chemical operating principles or a combination of the two operating principles.
Processes are further known from DE 4042009 C2, EP 0493732 B1, U.S. Pat. No. 5,597,851 and DE 19728036 A1 for microbial and enzymatic activation of powdered rubber and rubber granulate.
A process is disclosed in the documents DE 4042009 C2 and EP 0493732 B1 which is based on microbial oxidation of the polysulfide bonded sulfur in rubber vulcanization. The oxidation of the polysulfide sulfur on the surface of the rubber particles takes place by means of chemolithotropic microorganisms in a bacterial suspension with a defilled supply of oxygen. The bacteria belong to the Thiobacillus genus. The oxidation of the sulfur is generally carried out up to the sulfate stage. The end product of the process is a replasticized, low-sulfur rubber material with good suitability for vulcanization.
A similar process is disclosed in U.S. Pat. No. 5,597,851. The particularity of this process consists, on the one hand, in that the thermophilic optionally chemolithotropic Sulfolobus acidocaldarius is primarily used as a sulfur oxidizing microorganism and, on the other hand, the treatment of the rubber particles is carried out merely with the enzyme system of this microorganism. The rubber particles themselves are not in direct contact with the microorganisms.
A process is disclosed in DE 19728036 A1 in which by biotechnological treatment of vulcanized rubber particles by means of defined reaction times/duration of oxidation, specific reactive functional groups in the form of hydroxyl groups, epoxy groups and carboxyl groups are produced on the particle surface. As a result, it is possible to vulcanize the activated powdered rubber and rubber granulate with different plastics, bitumens and other polymers. Bacteria of the Thiobacillus genus are also used for the microbial oxidation.
The previously known processes for microbial activation of powdered rubber and rubber granulate by sulfur oxidation comprise the following important disadvantages:                1. These activation processes are based on oxidation processes. In addition to the desired oxidation of the polysulfide sulfur, undesired oxidation of the polymer chains (attachment of free radicals) inevitably takes place simultaneously. The points on the particle surface which are still bonding-active are practically eliminated. The degree of degradation depends, amongst others, on the type of rubber (number of double bonds), the reaction temperature, the duration of the reaction and the concentration of dissolved oxygen in the suspension.        2. The degradation to the polymer chains causes, amongst others, an undesired release of specific rubber constituents (plasticizers, carbon black, zinc oxide, etc.).        3. In order to avoid foreign contamination, the processes have to be carried out at very low pH values (1 to 3) which necessitates additional requirements for the materials of the bioreactors and for the waste water treatment.        
These disadvantages can be avoided by anaerobic processes. Such a process is known from Bredberg (K. Bredberg, J. Perssom, M. Christiansson, B. Stenberg, O. Holst: Anaerobic desulfurization of ground rubber with the thermophilic archaeon Pyrococcus furiosus—a new method for rubber recycling in the journal Appl. Microbiol. Biotechnol. (2001) 55, pages 4348), by using the sulfur-reducing, anaerobic, hyperthermophilic archaeon, Pyrococcus furiosus. This process nevertheless has the following disadvantages—in particular due to the hyperthermophilic characteristic of the archaeon:                1. The treatment of the powdered rubber over a lengthy period at a temperature range of 90-] 00° C. leads to degradation of the polymer chains of the elastomers and thus to deterioration of the significant material technical parameters (tensile strength, elongation at break, abrasion, etc.).        2. Due to the high temperature exposure of the powdered rubber, increased rubber constituents are released (plasticizers, carbon black, zinc oxide, chemical protective agents, etc.) which have a toxic effect on the microorganisms and thus restrict the process of desulfurization or lead to a breakdown of the process.        3. Carrying out the process at such a high temperature range is uneconomical with regard to large-scale production and of ecological concern (release of toxic materials into the process waste water).        